I would say as in infant she can wear anything. At 6months + maybe not but around 3 months a sleeper is fine

I wouldn’t be too concerned what to put on her, but if you want something special then that is fine. You might plan something she can wear a simple white onesie under and let her travel in the onesie and pair of pants. Then you can easily slip off the pants and dress on 👍🏼 plus you are half way to a comfy outfit when you are ready to relax!

When my father in law passed away in September my little guy just wore a sleeper but he was a month old and he slept the whole time. Also I would say yes same concern with a puffy dress as a jacket, but depends on how puffy. Do a test run before put her in the dress and Buckle her in see what It looks like to check safety.